Side Reactions and the Role of Thiosulfate

The side reactions can be divided into two categories: (a) reactions involving lignin,

carbohydrate and their degradation products; and (b) reactions involving inorganic

sulfur compounds only. All side reactions (Scheme 4.54) have in common

the fact that they diminish the available sulfite concentration and hence destabilize

the cooking liquor. Hydrogen sulfite in aqueous solutions normally acts as a

reducing agent and antioxidant. However, under the conditions of the sulfite cook

a major part of hydrogen sulfite is consumed by the reducing end groups of sugar

monomers and other keto groups present in the liquor under formation of ahydroxysulfonates

and subsequent oxidation of the reducing end to the corre-

422 4 Chemical Pulping Processes

sponding the aldonic acids, according to Scheme 4.55. The hydrogen sulfite

bound as a-hydroxysulfonate is classified as “loosely bound sulfur dioxide”.

The tendency to form a-hydroxysulfonates and their stability depend on the

type of the parent carbonyl compound. Hexoses, pentoses, and lignin carbonyls

form less-stable adducts as compared to formaldehyde, furfural, or methyl glyoxal.

Formic acid is converted to carbon dioxide [48]by sulfite.

HSO3

S2O3

S4O6

OH

HO

HO

OH

OH

COOH

O

HO

HO

OH

OH

OH

H

+